Lasers are widely used in the manufacture of advanced printed circuit boards (PCBs). A particularly well known example is the drilling of blind contact holes, so called micro-vias, in multi-layer PCBs. In this case ultra violet (UV) solid state lasers are often used to drill through a top copper layer and an underlying dielectric layer to allow contact to be made to a lower copper layer. In some cases, the cost effectiveness of this process is improved by using two different laser processes to remove the two different materials. A UV diode pumped solid state (DPSS) laser is usually used to drill the holes in the top copper layer to expose the lower dielectric layer and in a separate process a CO2 laser is used to remove the dielectric material exposed below each hole.
Recently a new type of high density multi-layer circuit board manufacturing technology has been proposed. US2005/0041398A1 and publication “Unveiling the next generation in substrate technology”, Huemoeller et al, 2006 Pacific Micro-electronics Symposium describe the concept of “laser-embedded circuit technology”. In this new technology, lasers are used to directly ablate fine grooves, larger area pads and also contact holes in organic dielectric substrates. The grooves connect to the pads and contact holes so that, after laser structuring and subsequent metal plating, a first layer consisting of a complex pattern of fine conductors and pads embedded in the top surface of the dielectric layer is formed together with a second layer consisting of deeper contact holes connecting to a lower metal layer. More information on the progress of this new technology was presented in papers EU165 (David Baron) and TW086-2 (Yuel-Ling Lee & Barbara Wood) at the 12th Electronic Circuit World Convention in Taiwan, Nov. 9th-11th 2011.
Up to now pulsed UV lasers have been used in such methods to form the grooves, pads and contact holes in a single process using either direct write or mask imaging methods.
The direct write approach generally uses a beam scanner to move a focussed beam from a laser over the substrate surface to scribe the grooves and also create the pad and contact hole structures. This direct write approach uses a highly focusable beam from a UV diode pumped solid state (DPSS) lasers with high beam quality and hence is very well suited to the fine groove scribing process. It is also able to deal well with the different layer depth requirements associated with pad and contact hole structures. By this method, grooves, pads and contact holes of different depth can be readily formed. However, because of the limited laser power available from highly focusable UV DPSS lasers, this direct write process is slow when it comes to removing the more substantial volume of material associated with the larger area pads and ground planes. This direct write method also has difficulty maintaining constant depth at the intersections between grooves and pads. A description of direct write laser equipment suitable for making PCBs based on embedded conductors was presented in paper TW086-9 (Weiming Cheng & Mark Unrath) at the 12th Electronic Circuit World Convention in Taiwan, Nov. 9th-11th 2011.
The mask imaging approach generally uses a UV excimer laser to illuminate a mask containing the full detail of one layer or level of the circuit design. An image of the mask is demagnified onto the substrate such that the full area of the circuit on that layer is reproduced on the substrate with a laser pulse energy level sufficient to ablate the dielectric material. In some cases, where the circuit to be formed is large, relative synchronized motion of the mask and substrate is used to transfer the full pattern. Excimer laser mask projection and associated strategies for covering large substrate areas have been known for many years. Proc SPIE 1997, vol. 3223, p 26 (Harvey & Rumsby) gives a description of this approach.
Since the whole area of the mask is illuminated during the image transfer process, this approach is insensitive to the total area of the individual structures to be created and hence is well suited to creating both the fine grooves, the larger area pads and ground planes. It is also excellent at maintaining depth constancy at the intersections between grooves and pads. However, due to ablation rate dependence on structure size, control of depth to high precision over all features is often difficult. Except in the case where the circuitry is extremely dense, this mask imaging approach is also significantly more costly than the direct write approach since the purchase and operating costs of excimer lasers are both very high. Mask imaging is also very inflexible in that a new mask needs to be used for each layer of the circuit.
The latter limitation is overcome in the arrangement described in publication US 2008/0145567 A1. In this case, an excimer laser scanning mask projection system is used to form a layer consisting of grooves and pads to the same depth in the insulating layer and, in a separate process, using a second laser which is delivered by a separate beam delivery system, the deeper contact holes penetrating to an underlying metal layer are formed. This two-step process is a way of dealing with the varying depth structure requirements. However, it still suffers from the high cost associated with the use of excimer lasers.
Hence, it can be seen that the existing process methods for making advanced circuits based on this “laser-embedded circuit technology” have serious disadvantages. There remains a need to be able to use laser processes that are optimized for creating the different size and depth structures required in a very flexible way, to improve the process rate and reduce the cost. The present invention aims to provide apparatus and a multistage stage process that retain the advantages of mask imaging but avoids the use of costly excimer lasers to address these needs.
According to a first aspect of the invention, there is provided apparatus for forming fine scale structures in the surface of a dielectric substrate to two or more depths, the apparatus comprising:
a first solid state laser arranged to provide a first pulsed laser beam; a first mask having a pattern for defining a first set of structures at a first depth, a projection lens for forming a reduced size image of said pattern on the surface of the substrate and a beam scanner arranged to scan said first pulsed laser beam in a two-dimensional raster scan relative to the first mask to form a first set of structures at a first depth in the substrate, and the first or a further solid state laser arranged to form a second set of structures at a second depth in the substrate.
According to a further aspect of the invention, there is provided a method of forming fine scale structures in the surface of a dielectric substrate to two or more depths, the method comprising a two-stage process: a first process which defines a first set of structures at a first depth and a second process which defines a second set of structures at a second depth, the first process comprising: using a first solid state laser to provide a first pulsed laser beam; providing a first mask having a pattern for defining a first set of structures at a first depth, providing a projection lens for forming a reduced size image of said pattern on the surface of the substrate and scanning said first pulsed laser beam in a two-dimensional raster scan relative to said mask to form a first set of structures at a first depth in the substrate,
the second process comprising use of the first or a further solid state laser to form a second set of structures at a second depth in the substrate, in which the first and second processes can be carried out in either order.
The invention thus involves a method which uses a solid state laser (SSL), a first process which involves 2D scanning of the laser beam (e.g. in the form of a laser spot) over a mask to form a first set of structures in the surface of a dielectric substrate and a second process (which can be carried out in a variety of ways) to form a second set of structures in the dielectric substrate.
The choice of laser enables the scanning to be carried out at high speed so the structures can be formed in a short time period whilst avoiding the need for high capital cost and/or high operating costs. The process also allows each of the process steps for forming the different types of structures to be separately optimised.
Key preferred features of the invention are:—                1) A method for forming sets of fine scale structures which, after a subsequent copper plating process, form an “embedded conductor” based electronic circuit layer being part of a multi-layer, high density, electrical interconnection device,        2) Sets of structures being formed in the surface of a dielectric material to two or more different depths, each depth being achieved using a separate laser ablation process,        3) A first process that creates a first set of structures to a first depth in the following way                    a. a first mask defines the first set of structures;            b. a projection lens forms a reduced size image of the first mask on the substrate surface;            c. a beam scanner unit moves the beam from a first Q-switched CW diode pumped solid state (DPSS) laser over the mask in a 2D raster pattern;            d. the energy density in the laser beam at the substrate on each laser pulse is sufficient to ablate the material of the substrate but not to damage the mask;            e. the laser beam spot size and shape at the mask coupled to the trajectory of the 2D motion of the beam over the mask surface are such that the first set of structures is defined to a uniform first depth over the full area of the device on the substrate,                        4) a second process that creates a second set of structures to a second depth over all or part of the device, the second set of structures being superimposed on some or all of the first set of structures so that the second depth is greater than the first depth. The second process uses one of the following methods:                    a. It repeats the first process using the first laser and the first mask to define the second set of structures but using different laser parameters to the first process. This could be the case when the substrate is layered with different materials or has a protective or sacrificial layer applied to the top side as shown in FIG. 3. In this case, the first process laser patterns the top layer of material or the protective/sacrificial layer and the second process laser patterns the material layer below;            b. It repeats the first process using the first laser but with a second mask to define the second set of structures;            c. Using the first mask with the beam from the first laser moved by the scanner to selected areas of the first mask and the first laser fired for a number of pulses such that those selected areas are subject to sufficient additional pulses to create a second set of structures having a second depth;            d. Using the first mask with an aperture in the first laser beam, the first laser beam being moved by the scanner to selected transparent features on the first mask, the aperture being imaged onto the mask such that the image on the mask is smaller than the transparent feature and the first laser fired such that those selected areas are subject to additional laser pulses and hence a second set of structures are formed to a second depth within and smaller than the first set of structures;            e. Using the first mask with a second laser which is different to the first laser, the beam from the second laser being moved by the scanner to selected transparent features on the first mask and controlled such that the beam on the mask is smaller than the transparent feature and the second laser fired such that those selected areas are subject to additional laser pulses and hence a second set of structures is formed to a second depth within and smaller than the first set of structures,                        5) A further process, if required, to create a further set of structures to a further depth over all or part of the device, the further set of structures being superimposed on some or all of the first or second set of structures so that the further depth is greater than the first or second depth. The further process uses one of the methods described above in sections 4.a-4.d,        6) In some cases, the first process may follow the second process. For example, second processes 4.c, d and e could all take place before the first process and the result in terms of the formation of two sets of structures to two depths in the substrate is the same.        
The mask used for the first and second processes may comprise a 2D array of pixels whose transparency to the laser beam can be changed electronically so that the full mask pattern can be changed dynamically.
Other preferred and optional features of the invention will be apparent from the following description and from the subsidiary claims of the specification.